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                <time datetime="2018-01-01T14:50:28.000Z" itemprop="datePublished" class="page-time">
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            <ol class="post-toc"><li class="post-toc-item post-toc-level-4"><a class="post-toc-link" href="#1-switch中case最好加"><span class="post-toc-number">1.</span> <span class="post-toc-text">1.switch中case最好加{}</span></a></li><li class="post-toc-item post-toc-level-4"><a class="post-toc-link" href="#2-范围for语句"><span class="post-toc-number">2.</span> <span class="post-toc-text">2. 范围for语句</span></a></li></ol>
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<article id="post-控制语句"
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        <h1 class="post-card-title">控制语句</h1>
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            <time class="post-time" title="2018-01-01 22:50:28" datetime="2018-01-01T14:50:28.000Z"  itemprop="datePublished">2018-01-01</time>

            


            
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            <h4 id="1-switch中case最好加"><a href="#1-switch中case最好加" class="headerlink" title="1.switch中case最好加{}"></a>1.switch中case最好加{}</h4><p><strong>目的</strong>：<strong><em>防止跨过变量的初始化语句（包括隐式和显示）直接跳转到该变量作用于内的另一个位置。</em></strong></p>
<p>case加{}：在该case中定义的变量，有效范围为该case；<br>case不加{}：在该case中定义的变量，有效范围是switch；</p>
<p><strong>错误案例：</strong></p>
<pre><code class="cpp"><span class="keyword">case</span> <span class="number">1</span>:<span class="comment">//假设控制流绕过了该case</span>
    <span class="built_in">string</span> file_name;<span class="comment">//错误：控制流绕过一个隐式的初始化变量</span>
    <span class="keyword">int</span> ival = <span class="number">0</span>;    <span class="comment">//错误：控制流绕过一个显示的初始化变量</span>
    <span class="keyword">int</span> jval;        <span class="comment">//正确：该变量没有初始化</span>
    <span class="keyword">break</span>;
<span class="keyword">case</span> <span class="number">2</span>:
    jval = next_num();      <span class="comment">//正确：给jval赋一个值</span>
    <span class="keyword">if</span>(file_name.empty()){} <span class="comment">//file_name在作用于中，但是没有初始化</span>
    <span class="keyword">break</span>;
</code></pre>
<p>假设上述代码合法，则一旦控制流直接跳到分支2，也就同时略过了变量file_name和ival的初始化过程。<br>此时这两个变量为作用于之内，跟在分支2中的代码试图在尚未初始化的情况下使用他们，这显然是行不通的。</p>
<p><strong>解决方案：</strong>如果某个case要定义并初始化一个变量，我们应该保证该变量在块内，防止其他case引用该变量。注意break在{}之外！</p>
<p><strong>正确案例：</strong></p>
<pre><code class="cpp"><span class="keyword">case</span> <span class="number">1</span>:
    {
        <span class="built_in">string</span> file_name = get_file_name();
    }
    <span class="keyword">break</span>;<span class="comment">//注意break在{}之外！！！！！！！！</span>
<span class="keyword">case</span> <span class="number">2</span>：
    <span class="keyword">if</span>(file_name.empty()){}<span class="comment">//错误：file_name不在作用于之内</span>
</code></pre>
<h4 id="2-范围for语句"><a href="#2-范围for语句" class="headerlink" title="2. 范围for语句"></a>2. 范围for语句</h4><p><strong>形式</strong>：<strong><em>for (declaration : expression) { statement}</em></strong></p>
<p><strong>注意事项</strong>：</p>
<ul>
<li>expression必须是一个序列：数组、vector或者string等包含begin()和end()的</li>
<li>每次迭代都会重新定义一个循环控制变量</li>
<li>范围for中预存了end()，一旦序列中添加（删除）元素，end()函数可能就变得无效</li>
</ul>
<pre><code class="cpp"><span class="built_in">vector</span>&lt;<span class="keyword">int</span>&gt; v = {<span class="number">0</span>,<span class="number">1</span>,<span class="number">2</span>,<span class="number">3</span>,<span class="number">4</span>,<span class="number">5</span>};
<span class="keyword">for</span>(<span class="keyword">auto</span> &amp;r : v){<span class="comment">//这里循环变量用的是引用，只有这样才能对元素执行修改操作</span>
    r += <span class="number">2</span>;
}
</code></pre>

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